The present invention relates, in general, to mixing and holding vessels and, more particularly, to an apparatus that redirects fluid flow through an exit port of a vessel for reducing or eliminating vortex formation as a fluid is drained from the vessel.
Various means for mixing fluids are known in the art. Both intrusive and non-intrusive means have been used to mix fluids, including colloidal suspensions, to prevent separation of homogeneous solutions into constituent components and/or to reconstitute solutions that have separated into constituent elements. Intrusive mixing devices, or those objects and devices which are inserted into a fluid to agitate the fluid with the assistance of an external power source, are well known. Such devices involve the use of intrusive mechanical mixers powered by electric or pneumatic motors. These devices provide relatively high torque and/or rotation of the fluid and may result in adverse effects on the fluid as a result of the formation of a significant vortex or whirlpool in the fluid. Moreover, when a fluid is drained from a holding vessel through a drain in a vertical direction, typically, pockets of little or no fluid movement may be created at the base of the holding vessel.
In some chemical environments, further adverse effects of intrusive agitation can be seen in the form of foaming or gelling of the body of fluid while it is being mixed in a mixing tank or similar holding vessel. Such foaming or gelling may change the parameters of fluids"" various chemical compositions and adversely affect their performance. Additionally, intrusive mixing devices and methods may introduce air into the mixture or fluid and may cause oxidation of certain chemical mixtures thereby changing the chemical reactivity of the fluid.
Fluids, and in particular, colloidal suspensions such as slurries used in Chemical Mechanical Planarization (CMP) of semiconductor wafers are most effective when delivered to CMP tools in a homogenous state with no air in the supply line delivering fluid to these tools.
In one embodiment the invention provides a whirlpool reduction cap comprising a top solid surface greater than or equal to the area of an exit port in a vessel, a base connected to the exit port, a side wall positioned between the top surface and the base. An inlet is positioned in the side wall which may comprise one or more orifices.
The invention has particular applicability for mixing and delivery of colloidal suspensions, including slurries used in CMP of semiconductor wafers. Such colloidal suspensions are notorious for separating from homogeneous distribution into constituent chemical components. More generally, however, the invention may be used in numerous other applications requiring homogeneous fluids, and it is not contemplated that the invention would be limited to slurry or CMP applications.